A type of sensor known from the related art is a hot-film air mass meter (HFM), an embodiment of which is described in German Patent Application No. DE 196 01 791, for example. In hot-film air mass meters of this type, a thin sensor diaphragm is typically applied to a sensor chip, which is preferably a silicon sensor chip. At least one heating resistor, which is enclosed by two or more temperature measuring resistors, is typically situated on the sensor diaphragm. The temperature distribution which may in turn be detected by the temperature measuring resistors changes in an air flow which is guided over the diaphragm. Thus, for example, an air mass flow may be determined from the resistance differential of the temperature measuring resistors. Various other variations of this sensor type are known from the related art.
A sensor chip is described in German Patent No. DE 101 11 840 which has a frame element manufactured from silicon, having a diaphragm applied thereto. Various metal webs which function as electrical heaters and/or measuring resistors are situated on the diaphragm, resulting in the area of the diaphragm forming a sensor area. Moreover, at least one auxiliary heater is additionally situated on the surface of the sensor chip and may be electrically heated in such a way that thermal gradient eddies are formed in the flowing medium in the area of the auxiliary heater, which result in deposits of contaminants in the area of the auxiliary heater beyond the sensor area and do not additionally contaminate this sensor area.
To detect hydrogen, the property of hydrogen of having significantly better thermal conductivity than air is exploited. In a sensor construction which is similar to that of the hot-film air mass meters (HFM) outlined above, an air-hydrogen mixture, for example, diffuses through a thin diaphragm or a tight grid into the measuring chamber. The presence of hydrogen changes the temperature of the heated measuring diaphragm or its thermal output, which is delivered to the surrounding air, which in turn results in the measurement signal. In these embodiment variations, the measuring chip and/or housing temperatures are typically at approximately room temperature, diaphragm temperatures typically being set between 80 K and 120 K.
This measuring method has the disadvantage that moisture contained in the gas mixture influences the thermal conductivity of an H2-air mixture. At room temperature, the influence of the moisture component may be so large that detection of hydrogen in the H2-air mixture is no longer possible with the required clarity.